The present invention relates to fast breeder reactors, and more particularly, it relates to structural components for constituting a reactor core, a core of reactor constituted by such components, and a method for operating such reactor.
In general, a core of a fast breeder reactor is formed by bundling a plurality of fuel assemblies each comprising a bundle of cladding tubes in which fuel pellets are filled respectively and each enclosed by a hexagonal duct, to provide a coolant flow channel. Further, in such reactor core, the breeding of the fuel is improved by enclosing the core by axial and radial blankets of fertile material. The fuel such as enriched uranium or plutonium added uranium is mounted in the core, and the fertile material such as natural uranium or depleted uranium is mounted in the blankets. When the fertile material captures fast neutrons having high energy escaped from the core, useful fissile materials are produced. At the same time, the fast neutrons are absorbed in the members constituting the reactor core such as the cladding tubes and ducts, thus expelling atoms from such members and/or nuclear-reacting with impurity to create bubbles of helium gas, thereby swelling the material of such members. The life of the fuel in the fast breeder reactor is often limited by bundle-duct interaction (BDI) between the fuel bundle and the corresponding duct and/or duct-duct interaction (DDI) between the adjacent ducts, due to the swelling of material of the core constituting members. Similarly, in some cases, the life of a control rod which forms a part of the structural component constituting the reactor core is limited by the interaction between the absorber rod bundle and the corresponding duct.
The degree of the swelling of the core constituting members depends upon fast neutron fluence and/or irradiation temperature with respect to the fuel assemblies. FIG. 4 shows the axial distribution of these features (i.e., the fast neutron fluence and irradiation temperature) with respect to the fuel assembly in which the fast neutron fluence of the core in a homogeneous core of electrical rating of 1,000 MW is maximum. As apparent from FIG. 4, in the vicinity of the axial center of the reactor core where the fast neutron fluence is maximum, a cladding temperature differs from a duct temperature by about 100.degree. C. In the conventional design of the fast breeder reactors, in many cases, the cladding tubes were made of the same material as the ducts. With such construction, the BDI or DDI will occur owing to the difference in the swelling rates between the cladding tubes and ducts due to the difference in the irradiation temperatures between the cladding tubes and the ducts. That is to say, when the temperature at which the swelling rate of the material becomes peak or maximum (i.e., the peak swelling temperature) is in the vicinity of the range of the irradiation temperature of the cladding tube (cladding temperature), the BDI will occur since the swelling of the cladding is larger than that of the duct whose temperature is lower than the cladding. On the other hand, when the peak swelling temperature of the material is in the vicinity of the range of the irradiation temperature of the duct (duct temperature), the BDI will not occur since the swelling of the duct is larger than that of the cladding. But in this latter case, the DDI will occur.
In order to reduce the BDI, an improved fuel assembly comprising cladding tubes made of material having relatively small swelling rate and ducts made of material having relatively large swelling rate has been proposed, as disclosed in the Japanese Patent Laid-open No. 57-166591.
With such conventional fuel assembly, the BDI can be reduced or prevented since the swelling of the cladding is smaller than that of the duct. However, in such conventional fuel assembly, since the reduction regarding the swelling of the duct is not devised, when the fuel life is limited by the occurrence of the DDI, it is not expected that the fuel life is extended or increased.